Safety around oxygen
Spare parts in air separation plants

A blue liquid races through the network of pipes in an air separation plant. It is high-purity oxygen – cryogenically frozen to a liquid at minus 183 degrees Celsius. Dozens of process valves guide the liquid gas through the pipes and ensure that it arrives where it is needed for industrial and medical applications. “Pure liquid oxygen has to be handled with the utmost care and the highest cleanliness levels have to be maintained,” explains Johann Berger, one of the people responsible for the spare parts service at Linde Engineering. Any kind of contamination, even tiny particles, are a safety risk. “In an oxygen atmosphere, even a pressure surge in conjuction with non-compatible impurities could be enough to ignite a valve. This can then lead to explosive combustion, which can endanger people’s lives,” elaborates Berger.

Battery of tests for process valves

To prevent this from happening, the Linde expert and his colleagues make sure that spare parts meet the stringent standards required for safety in oxygen atmospheres. This interdisciplinary team was formed back in 2013 and tasked with developing new, stricter standards that would enable it to offer customers an even better service. As a result, the team painstakingly checks valves manufactured by different suppliers that come into contact with this strong oxidising gas before they are delivered to new or existing plants. As such, Linde acts as a control point and provides an additional layer of quality control between valve manufacturers and their customers – in other words, air separation plant operators. The plant specialists primarily focus on the valves and their individual parts such as rubber and Teflon seals. “There are no contaminants in the actual stream of oxygen. Contamination always comes from the parts installed in the plant,” explains Berger.

Specialists use a wipe test to find out if there are any unwanted particles or grease on the surface of valves.

All successful quality control programmes are built on end-to-end documentation. That is what enables the Linde experts to know for sure if the materials used in the valve are suitable for oxygen service – which is why they also request documentary proof of this from manufacturers. “We request cleaning records and documents confirming that these parts are qualified for contact with oxygen and have been tested for oxygen service,” says Berger. The experts then carry out a visual inspection of the valves in bright light at Linde's site in Pullach to ensure that they are clean and made of suitable materials. They then look at them under UV light to check for undesirable organic contaminants. The wipe test shows them whether there are any contaminants such as fat residue or dust particles on areas that cannot be directly accessed. “If the tests reveal clear indication of contamination, we send the valve or its spare parts back to the manufacturer for further cleaning,” adds Berger.

UV light reveals contamination that is difficult to detect under normal lighting conditions. Linde Engineering painstakingly checks valves that come into contact with oxygen for even the slightest traces of contamination.

All-round service for greater safety levels

He and his colleagues try to make manufacturers aware of the higher standards in this area. “We know that we demand very high levels of cleanliness. But it is the only way that we can guarantee the safety of our air separation plants,” says the Linde expert. This is because every plant contains dozens of valves and each one has its own replacement timeframe. Components continually need to be replaced and we cannot allow this to endanger plant safety. Nowadays, Berger increasingly receives concrete questions from customers who need a specific process valve for the oxygen section of their plant. “We use our documentation to see if it comes into contact with oxygen, find out which manufacturer can deliver it – to our current quality standards. We then procure and check the component,” explains Berger. If the process valve passes the battery of tests, it is repackaged by Linde Engineering, sealed to keep it airtight, given an appropriate seal of approval and sent to the plant operator. “We also offer to have our experts install the component on site,” says Berger, explaining the last facet of Linde’s all-inclusive oxygen service offering. Linde has built over 3,000 air separation plants worldwide. “Our end-to-end service package plays a key role in ensuring the continued safe and reliable operation of these plants over time,” concludes Berger.

The cleaner the better: Oxygen valves have to be totally free of contaminants to be suitable for service in air separation plants.